There have been examined the radar devices which use millimeter waves or quasi-millimeter waves to realize high-accuracy position determination, aiming for collision prevention in automobile traffic and the like. One example of such radar devices is a pulse radar device which transmits pulse signals by a transmission antenna and detects waves reflected at an object by a receiving antenna. This pulse radar device determines a distance from and a position of the object by calculating a delay difference between the transmitted pulse signal and the received pulse signal.
In such a radar device, isolation between the transmission antenna and the receiving antenna is crucial. The isolation between the transmission antenna and the receiving antenna means a degree of leakage or interference of waves or signals between the transmission antenna and the receiving antenna. The isolation providing less leakage or interference is considered as good isolation.
When signals transmitted from the transmission antenna is leaked into the receiving antenna, a receiving unit which judges signals received by the receiving antenna cannot distinguish the leaked signals from signals reflected at an object. As a result, the leaked signals become noise in the receiving unit, and the receiving unit has a difficulty in detecting the signals reflected at an object. For radar devices, radio field intensity of received waves is quite lower than radio field intensity of transmitted waves. This is because waves which are reflected at an object and received by a radar device are attenuated in proportion to a power of 4 of a distance from the object. For example, when transmitted waves are reflected at a human body 10 m ahead and then return, an attenuation amount of the reflected waves is approximately −90 dB.
A distance within which a radar device can detect an object depends on how much isolation can be established between a transmission antenna and a receiving antenna. Therefore, the isolation between a transmission antenna and a receiving antenna is the most important characteristic to decide radar efficiency.
In recent years, size reduction and low cost have been demanded for radar devices. In order to meet the demand, there has been proposed a radar device in which thin planar microstrip antennas are used as antenna elements and a transmission antenna and a receiving antenna are formed on the same substrate (refer to Patent Reference 1, for example).
FIG. 1 is a plan view showing a structure of a conventional radar device.
The radar device shown in FIG. 1 includes a transmission antenna 1301, a receiving antenna 1302, and a ground conductor 1303.
The ground conductor 1303 is arranged between the transmission antenna 1301 and the receiving antenna 1302, and is electrically connected to ground. By forming the ground conductor 1303, the conventional radar device improves isolation between the transmission antenna and the receiving antenna.
[Patent Reference 1] Japanese Unexamined Patent Application Publication No. 2005-94440